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In ScopedCrosSettingsTestHelper, there was one call to
CrosSettings::Get()->Set - this was just used for testing to set
any CrosSetting to a particular value.

If every call to CrosSetting::Get()->Set can be removed, that
means we can delete the entire write path in DeviceSettingsProvider,
which will make code simpler. That means all writes will be using
OwnerSettingsService, which is better because it is profile-aware.

In practise, most tests are using a StubCrosSettingsProvider,
so they don't need to use the write path of DeviceSettingsService,
or the OwnerSettingsService - they can just write it directly
to the StubCrosSettingsProvider.

This test modifies ScopedCrosSettingsTestHelper so it can also
use the StubCrosSettingsProvider when it was initialized by
CrosSettings itself. It also modifies 2 tests that were not
using a StubCrosSettingsProvider, but need to call Set, so that
now they are using a StubCrosSettingsProvider.

For more background, see go/writepath

Bug: 433840
Change-Id: I85a68d6ea116061b13f3c5e5582aa0298af1a386
Reviewed-on: https://chromium-review.googlesource.com/c/1299002Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: A Olsen <olsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603530}